Mechanical/thermal dewatering of lignite. Part 3: Physical properties and pore structure of MTE product coals

Abstract

The mechanical thermal dewatering (MTE) process has been shown to effectively dewater high moisture content low rank coals via the application of mechanical force at elevated temperatures. Using mercury intrusion porosimetry (MIP) as an investigative tool, this study examines how MTE processing conditions, such as temperature and pressure, affect the compressibility, pore size distribution, apparent (skeletal) density and shrinkage behaviour of three low rank coals sourced from Australia, Greece and Germany. As both pore filling and sample compression occurred at high mercury intrusion pressures, all MIP data were corrected for compression effects by using compressibility values derived from mercury extrusion data. The MTE process is shown to produce a low porosity coal, which, depending upon the processing conditions used, undergoes further shrinkage upon oven drying at 105 °C. An increase in MTE temperature (above about 85 °C) led to an increase in mesopore volume, which is caused by a hardening of the coal structure, leading to pore volume retention and a consequent reduction in percent shrinkage on oven drying. The increase in measured mesopore volume is also associated with an increase in measured surface area. The reverse trend is seen with increasing MTE pressure, where both the macro and mesopore volume decrease with pressure, causing the percent shrinkage to increase accordingly. This effect may be due to an increase in capillary forces caused by a decrease in the average pore diameter. The percent shrinkage increased up to a pore volume of about 0.1 cm 3/g, beyond which no further reduction in pore volume was achieved. The decrease in mesopore volume is also associated with a decrease in measured surface area. Compressibility values derived from mercury extrusion data show that the MTE process has little impact on the network strength of the skeletal network structure of all three coals investigated. Likewise, the skeletal density remained relatively unchanged. The reduction in water content, pore volume and the changes in shrinkage behaviour under increasingly severe MTE conditions are suggestive of the physical changes that accompany increased coalification (rank) within the lignitic range.